Using viruses to deliver the genes, she inserted all 14 at once into human cells. On the morning of July 1, 2006, Yu arrived at the lab and examined the culture dishes. Her eyes focused on a few colonies, each resembling a crowded city viewed from space. They looked like embryonic stem cells.
…
Cells must pass certain tests. They must multiply for weeks while remaining in their delicate, primitive state. When they are allowed to develop, they must turn into all the other cell types.

Bad things happen. Cells develop too soon. Cells die. There is no “aha!” moment, Thomson has said, only stress. He looked at the colonies and suppressed any excitement. He told Yu, essentially: OK, well get back to me in a couple of weeks.
…
In the fall of 2006, Yu was preparing to whittle down her list of genes when she fell ill. The pain in her gut was awful. She struggled to eat. Her doctor thought it was a stomach flu. Instead, in late October, Yu’s appendix burst. She was laid up for a month. When she returned to the lab, the problem with the culture medium struck again.

Not until January 2007 was she able to begin narrowing the list of genes. She spent several months testing subsets of them, finally arriving at four. Two, Oct4 and Sox2, were “Yamanaka factors,” the name given to the genes the Japanese scientist had used to reprogram mouse cells. Two, Nanog and Lin28, were not.

Using a virus to deliver the four genes, she reprogrammed a line of fetal cells, then repeated the experiments with more mature cells. Although the process was inefficient, succeeding with only a small fraction of cells, it did work.